Wetlands are important systems of water environment, significantly
contributing to sustainable river basin management and maintaining good
ecological quality of water bodies (EC 2000). The importance of these
landscape elements and the need to protect them are also emphasized in
other international documents. For example, one of the main purposes of
the Ramsar convention is to stop the decline of palustrine, riverine and
lacustrine aquatic ecosystems as well as to ensure the protection of
these valuable natural complexes (Convention ... 1971).

Farming activities and climate change (Acreman et al. 2009;
Kovarova & Pokorny 2010) are among the most critical contemporary
issues that affect the water quality and quantity of aquatic ecosystems.
These changes influence the structure of aquatic habitats that are
important for waterfowl as breeding, nursery and protective areas. One
of the main threats to these ecosystems is extraction of natural
resources (for example, dolomite) in their surroundings. The water level
changes caused by such anthropogenic activities could negatively affect
the structure of aquatic habitats.

The idea of predicting water regime changes in aquatic habitats
arose during the environmental impact assessment for a dolomite mining
project (Environmental ... 2008). According to this assessment, there
were no reliable data that could confirm a likely influence of the
proposed dolomite mining on the changes of the Lake Cedasas level.
Therefore an independent investigation was initiated. A unique
methodology for evaluation of the optimal habitat structure for
waterfowl was created, which afterwards was applied to the particular
case of Lake Cedasas.

Understanding the optimal aquatic habitat structure for waterfowl
and its possible alteration after water level changes is a fundamental
ecohydrological question. Moreover, a negative influence of dolomite
mining could play a crucial role in the NATURA 2000 sites--the protected
habitats of European importance. Therefore, it was hypothesized that the
decline of groundwater level in the dolomite quarry and the predicted
climate change would affect the water level in the lake, situated in the
surroundings of the quarry. Change in the hydrological regime of the
lacustrine aquatic ecosystem would result in a rapid succession of
changes in vegetation, leading towards the destruction of waterfowl
habitats. The water level determined by groundwater inflow from aquifers
is of crucial importance for waterfowl habitats. Thus a decrease in the
water level in these aquifers would cause a dangerous lowering of the
lake water level during a dry season.

Moreover, the Little Crake (Porzana parva) and the Black Tern
(Chlidonias niger) species are preserved in the study area. The quality
of habitats of these birds is directly related to the hydrological
regime of the aquatic environment. Therefore the main goal of this
article is to assess changes in wetlands serving as important waterfowl
habitats, situated in the impact zone of proposed dolomite mining.

The objectives of this study were to (1) evaluate the water regime
and the vegetation cover on a selected territory in the problematic
region, (2) determine the hydrological indicators whose change could
threat the existence of sub-wetlands in the NATURA 2000 area (a shallow
lake and its riparian zone), (3) simulate the anticipated changes in the
lake and the groundwater level in the riparian zone according to the
FEFLOW 5.0 program and (4) determine the expected changes of sub-wetland
types according to the simulated groundwater level and lake depth.

METHODS

Study area

Lake Cedasas, situated in a sub-watershed of the Vyzuona River
(3rd-order river in the Lielupe watershed), was chosen for this study.
The length of the Vyzuona River is 34.1 km and its watershed area is 32
090 ha. The Minava Stream, flowing from Lake Cedasas to the Vyzuona
River, enters it about 23 km from the river source. The proposed
dolomite mining site is situated near this section of the river, only at
a distance of about 0.4 km, near Cedasai settlement. Lake Cedasas is a
shallow eutrophic lake, with an area of 49.9 ha mostly overgrown with
helophytes and hydrophytes. The area of the lake watershed is 2580 ha
(Gailiusis et al. 2001), 65% of which is situated in Lithuania, while
the rest belongs to Latvia (Fig. 1). The average depth of the lake is
0.86 m, maximum depth 1.85 m and the water turnover rate is 23 days.

The only inflow of the lake, the Minava Stream with an agrarian
watershed (2420 ha), enters Lake Cedasas in its eastern part. The same
stream flows out of the southern part of the lake (Fig. 1). A solid weir
(i.e. non-regulated solid concrete construction) occurs 250 m downstream
from the lake, therefore there is no discharge from the lake at very low
water levels.

The absolute height of the wetland habitat is about 86 m a.s.l. The
height of the Earth's surface in the proposed mining area is
similar and varies in the range of 86-88 m a.s.l. The groundwater levels
of the wetland area (about 85 m a.s.l.) and the mining area (84.5-86.4 m
a.s.l.) are similar in the conditions of the natural groundwater regime,
however, they will differ after the opening of the dolomite quarry
affecting the groundwater regime (possible drawdown to less than 80 m
a.s.l. in the mining area). According to the existing project, the water
pumped out of the quarry will be directed to the Vyzuona River, which is
not connected with the wetland water system.

There was no net discharge from Lake Cedasas till the end of the
19th century (Report ... 2007). The amplitude of lake water level
fluctuations was about 0.6 m. Later on, after partial drainage of the
riparian zone and watershed of Lake Cedasas, its outflow became
seasonal. Water discharge took place only when the lake level was very
high and the sub-wetlands in the riparian zone were submerged at the
southern end of the lake--normally in spring and autumn. The riparian
zone sub-wetland accumulated a depth of about 200 mm of water during the
floods and protected the lake from serious declines of water level
during the dry season. However, the water surplus during the first half
of the period of vegetation growth influenced the formation of wetlands
in the riparian zone. The water turnover rate increased (especially
during the flood periods) and the water resources of the riparian zone
decreased after the deepening of the lake outflow, the construction of
the weir and the installation of the drainage system of open ditches in
the western part of the riparian zone in 1977 (Fig. 1).

The effect of such reconstructions is obvious during the dry summer
periods--there is no outflow from the lake and the evaporation from the
lake surface contributes to rapid decline of its water level. Nowadays
the input of water to the lake exceeds the loss only during the period
from March to June; the water balance of the lake is negative during the
rest of the year. A large amount of water enters the lake with the
surface inflow; the amount of water entering directly the lake surface
from precipitation is roughly similar to the groundwater inflow (Table
1). The greatest part of surface outflow takes place during
March-May--more than 62% of annual outflow. During the warm season, the
inflow to the lake can be smaller than both evaporation and outflow from
the lake. If the lake water level is declining to the weir level, the
outflow from the lake stops. Net seasonal discharge is typical of this
lake. However the lake water level can increase by up to 1 m during the
flood periods.

It is proposed that during the drainage of the quarry, groundwater
could be used for the technological processes, and its surplus could be
discharged to the drainage ditches nearby. The quantity of discharged
water could reach up to 42 L [s.sup.-1]--this is the approximate inflow
of groundwater in the quarry. During the environmental impact
assessment, a marginal groundwater level decrease of 0.5 m from the
initial level was estimated in the drainage area. Such a decrease should
not affect the surrounding groundwater users, the groundwater layer or
the bodies of surface water. However, it was recognized that groundwater
level decrease could be higher and have a negative influence upon the
lake water level and habitats. On the other hand, this earlier
assessment contained no data on how the protected waterfowl habitats
would respond even to a smaller groundwater decrease.

The amount of precipitation has decreased and the air temperature
increased in the watershed of Lake Cedasas and the surrounding region.
If this tendency persists, the water budget will change (Table 1) and
the water level of the lake will decrease by about 10 cm in future.

Terminology

Wetlands are heterogeneous landscape elements. According to their
vegetation structure and other characteristics, they can be divided into
several groups, types or subtypes. Usually the main classification
features of such systems are watershed characteristics, including
land-use or land-cover type (Detenbeck et al. 2000),
hydro-geo-morphological differences (Brinson 1993), vegetation type
(Grossman et al. 1998) or some combination of these features (Cowardin
et al. 1979). The nature of the problems to be solved will determine
what kind of classification is the most useful. Therefore, according to
the main goal of this paper, a modified Cowardin classification system
of lacustrine aquatic ecosystems is used.

According to the Ramsar Classification System for Wetland Type
(Information ... 2009), Lake Cedasas is considered as one wetland
type--permanent freshwater pools. However, the differences in vegetation
cover and hydromorphological parameters enable distinction of several
sub-wetlands, among those forming the entire habitat for protected
waterfowl. In order to evaluate the effect of water regime changes, five
sub-wetlands were distinguished based on the vegetation structure of
Lake Cedasas and its riparian zone: open water, hydrophytes, helophytes,
open grass and mire scrub (Table 2). All water areas without vegetation
were considered as open water sub-wetland. Such areas are important to
various protected bird species. For example, the Black Tern tends to
nest at a site with 50 : 50 vegetation cover : open water (Cuthbert
1954; Hickey & Malecki 1997; Mazzocchi et al. 1997; Naugle et al.
2000). Nesting occurs at water depths ranging from 0.5 to 1.2 m (Dunn
1979). Hydrophyte sub-wetland consists of areas covered with floating
vegetation such as spatterdocks (Nuphar lutea), white waterlilies
(Nymphaea odorata), floating-leaf pondweed (Potamogeton natans), etc.
This sub-wetland could be considered as a transitional one between the
open water and overgrown sub-wetlands. Helophyte sub-wetland consists of
territories covered with water plants with only the underpart submerged,
so that their stems with blooms emerge from the water: bulrushes
(Scirpus spp.), cattails (Typha spp.), etc. Cattails or bulrushes are
characteristically dominant in Black Tern colonies (Dunn 1979). Open
grass sub-wetland consists of territories that are covered with emergent
riparian grass vegetation for the greater part of the year. Here the
same plant species grow that can be found in helophyte sub-wetlands.
However, the predominance of lower herbaceous plants is obvious here.
Mire scrub sub-wetlands are characterized as temporarily (during floods)
submerged territories overgrown with scrubs. The most common plants of
these areas are white willows (Salix alba), bird cherries (Prunuspadus),
etc.

It is supposed in this work that the complex of these sub-wetlands
composes a necessary environment for protected waterfowl. Elimination or
drastic reduction of only one sub-wetland could disturb the whole of the
Lake Cedasas ecosystem. The main factor influencing the formation of
these sub-wetlands is the level of water in the lake and its
surroundings. The changes in this parameter will induce changes in the
sub-wetland areas (Table 2).

Remote sensing analysis

The limits of the areas covered by the above-mentioned sub-wetlands
were determined according to the orthophotographical map (M 1 : 10 000,
2005-2006) and LANDSAT 7 satellite telemetric data (LANDSATETM imagery).
The ERDAS IMAGINE program was used to work with LANDSAT ETM data.
Interpretation and analysis of remote sensing data involved both
multi-spectral ETM imagery (with 1, 2, 3, 4, 5 and 7 spectral channels)
with the resolution of 28.5 m and imagery with increased resolution of
14.25 m. Statistical filtering (one of the spatial enhancement
operations that enables improving image pixel values) and image contrast
enhancement operations were made in order to extract as much information
as possible. Sub-wetlands were classified using one of the unsupervised
training algorithm methods--RGB clustering. A thematic raster layer of
the investigated territory was formed. A combination of the information
found in both remote sensing data was used to form the whole picture of
Lake Cedasas sub-wetlands.

The limit between the open water and hydrophyte sub-wetlands was
distinguished from the orthophotograph.

Discerning the differences between the boundaries of the
territories covered with hydrophytes and helophytes was the most
difficult problem. Field observations during several years in Lake
Cedasas show that the limit between hydrophytes and helophytes is quite
stable. Therefore, the satellite image taken in early summer (10 June)
when the hydrophytes have not yet appeared and the lake is covered with
the previous year helophytes (reeds, cattails, etc.) enabled us to
distinguish the boundary between these two sub-wetlands.

Later on, the limits of sub-wetlands were examined in relation to
the depth of the lake, determined according to the Lake Cedasas
bathymetrical map of 1970. Using this map, we could draw the most
accurate shoreline. Afterwards, the limits of sub-wetlands, situated in
the riparian zone (open grass and mire scrub sub-wetlands), were
determined based on satellite imagery and orthophotographs.

The limits of these sub-wetlands were also compared with the
groundwater depth, determined from hydrogeological maps (M 1 : 50 000).
The vertical precision of the bathymetrical map is 1 cm, the horizontal
precision is 10 cm. The average density of the depth measuring points is
71 m. Finally, about 100 depth measuring points were made. Therefore,
the precision of the bathymetrical map is 2 measuring points in 1 ha.

The best correlations of sub-wetland limits (distinguished from the
orthophotograph and satellite image) with particular depths of the lake
or groundwater depth were investigated. The best match of these limits
was taken as the parameter characterizing each sub-wetland, i.e. the
depth of the lake and the groundwater became the so-called
hydromorphological indicators (Table 2). Sub-wetland areas were
calculated and compared using SURFER 8 software, which automatically
sets the optimal step of the grid (11 m). However, in order to get the
best visualization of isolines, the step of the grid of 8.2 x 8.2 m for
depth interpolation and 5.7 x 5.7 m for sub-wetland boundaries was
chosen. Sub-wetland areas calculated on the basis of the remote sensing
data were compared with habitat areas, calculated from the lake and
groundwater depth.

FEFLOW 5.0 modelling

The purpose of the following stage, modelling, was to estimate the
changes in lake water level and groundwater depth in the riparian zone
during the exploitation of the dolomite quarry. For this purpose, the
modelling software FEFLOW 5.0 was applied (Diersh 2002). This system has
been used for specific hydrological and hydrogeological conditions of
the studied watershed and for verification of some other parameters. It
has been applied to particular geometrical and hydraulic properties of
the modelled area of the Lake Cedasas watershed (Figs 2, 3). For
calibration and validation of the model, the materials of geological
investigation from the Lithuanian Geological Survey (Gasiuniene &
Gasiunas 1973; Report ... 2007) for the area of the planned quarry and
investigated lake watershed were used.

Hydrogeological setting

Two layers of the Upper Devonian period (D3kp, D3ss) occur under
lacustrine and glacigenic Quaternary deposits (Q1, Q2) (Fig. 3) in the
dolomite mining area and its vicinity. They are composed of
fine-crystalline, solid, in places porous, cracked and cavernous
water-bearing dolomite which stratigraphically is dated as the
Kupiskis-Suosa aquifer ([D.sub.3]kp-ss) containing two layers. The
general average thickness of the aquifer reaches about 13 m. The top of
the aquifer on the territory of the quarry lies at a depth of 3.5-9.6 m
or 78.2-83.7 m above sea level. During the geological survey, the depth
of appearance of this water and levelling-off in all boreholes has been
measured. The piezometric level of water was achieved in the range
83.7-85.2 m a.s.l. The pressure head of water varies from 1.9 to 4.4 m,
on average 3 m, above the top of the aquifer. In this way, the water is
subartesian, whereas the lower part of morainic loam (Q2) is an obvious
aquitard. On the other hand, the water of Quaternary formations is
theoretically associated with the [D.sub.3]kp-ss aquifer. This should be
taken into consideration when assessing the environmental impact of
quarry drainage, because vertical leakage of water may reduce the
distances of potential impact.

The [D.sub.3]kp-ss aquifer occurs above the Upper Devonian layer
([D.sub.3]jr) which consists of about 10 m thick clay and
clayey-carbonaceous deposits of low water permeability (hydraulic
conductivity [10.sup.-3]-[10.sup.-5] m [day.sup.-1]). The [D.sub.3]jr
layer can be considered as a regional aquitard which essentially
complicates vertical groundwater exchange between the underlying
terrigenous Midddle-Upper Devonian aquifer ([D.sub.3-2]sv-up). It was
considered (Report ... 2007) that, because of the rather small quarry
area and rather weak permeability of the aquitard, leakage of water from
the underlying aquifer will not have any practical significance.

Lake Cedasas lies on a shallow lowland and is surrounded by drained
wetland. The thickness of till in the vicinity of the lake is 5.5-6.5 m.
In the wetland situated in the northwestern part of the lake, the
average thickness of the turf layer is 2.7 m, in the southern part 1.55
m. Under the turf and at the bottom of Lake Cedasas, there occurs a
sapropel layer approximately 1.8 m thick. These layers are characterized
by extreme resistance to water release and low conductivity, so leakage
of water through the sapropel layer practically does not take place at
all. In addition, these layers overlie the 5-6 m thick low-permeable
floor of loam.

The geological-hydrogeological characteristics of the Cedasai
modelling area are presented in Figs 2 and 3 and Table 3.

Structure of the groundwater model

The main feature of the model of groundwater flow is
three-dimensional (3D) steady or transient (non-steady) flow. The area
of the model includes the entire lake watershed and the territory
between the lake and the planned dolomite quarry (Fig. 2). The geometry
of the model contains five layers, the lowermost of which is a regional
aquitard (layer 5; Fig. 3). The groundwater flow in all aquifers in a
numerical model is attributed to five layers (six slices) and is
represented by the 3D finite-element grid of mesh elements (Table 3).
The geometrical discretization of the model area and the set of boundary
conditions after the model calibration are presented in Fig. 4. The
total modelled area is 5.0618 x [10.sup.7] [m.sup.2] (Fig. 3), the
number of triangle elements (mesh) is 32 908, the number of units (mesh
node) is 21 350, the minimal step of the grid (mesh) which was used for
the area around the dolomite quarry and the investigated lake is 14-20 m
and the minimal area of the element is accordingly ~240 [m.sup.2].

Layer 2 (2-13 m thick) is composed of glacial loam (till) of low
water permeability, which can be considered as a relative aquitard
located above the underlying aquifer. Layers 3 (2-13 m thick) and 4
(2-13 m thick) consist of dolomite that corresponds to the Upper
Devonian aquifers (D3kp, D3ss). The last layer 5 (10 m thick) includes
clay and clayey-carbonaceous deposits.

The hydrogeological parameters after the model
calibration--hydraulic conductivity, recharge and storativity--are
presented in Table 4. They were used for simulation of the distribution
of the groundwater flow, representing the real water balance. For this
purpose, the published results of field investigations or archive data
bases were applied (Gasiuniene & Gasiunas 1973; Juodkazis 1979).

The following boundary conditions were used in the model (Fig. 4):
the 1st-kind condition, constant head--piezometric level of the Upper
Devonian aquifer (the pressure head 3-4 m above the top of the aquifer)
(layers 3 and 4); the 2nd-kind condition--constant groundwater flow at
the edge of the limitary watershed (layers 1 and 2); the 3rd-kind
condition--constant water exchange between rivers, lakes of the
watershed and adjacent computational blocks, transfer rate 5000/20 000 m
[day.sup.-1] x [10.sup.-4] (in/out) (layers 1 and 2); the 4th-kind
condition--extraction of water from the quarry with a constant yield of
158.3 [m.sup.3] [h.sup.-1] (in layer 3) (Report ... 2007). During the
selection of model parameters, the conditions were slightly and
intentionally adjusted to avoid over-optimism. A rather high hydraulic
conductivity of lower Quaternary loam (~0.001 m [day.sup.-1]) and rather
high conductivities of other layers were supposed. A more significant
contact between the Quaternary and the Upper Devonian aquifer can be
caused by the reduction (even up to 2 m) of the thickness of layer 2.
This means that the actual depression of groundwater levels caused by
the exploitation of the quarry could be slightly smaller.

Modelling with FEFLOW 5.0 was performed in two stages:

1. in the conditions of a steady state flow, before the beginning
of the dolomite quarry exploitation;

2. in the conditions of a transient (non-steady) flow, if pumping
of water with a maximum extraction of 158.3 [m.sup.3] [day.sup.-1] from
the Upper Devonian [D.sub.3]kp aquifer during exploitation of the quarry
were to be performed and continued for eight years.

The main result of the proposed modelling is the simulation of the
distribution of water levels between the groundwater and the Upper
Devonian [D.sub.3]kp aquifer in both cases mentioned above. In order to
represent the groundwater level isolines, a software SURFER-8 was used.

For the variability (sensitivity) analysis, runs of the model were
made for four sets of hydraulic conductivity, recharge, storativity and
transfer rate. The dependence of water level (after 8-year exploitation
of the dolomite quarry) on relative values of these four parameters in
the range from 0.1 to 10 is presented in Fig. 5.

The result of the assessment of the analysed parameters least of
all depends on the storativity and transfer rate. So, these parameters
are not critical and their variability cannot significantly affect the
results of modelling. The largest variability is determined for the
recharge (the difference between the upper and lower bounds for the
water level in the observation point No. 4 is 3.1 m) and for hydraulic
conductivity (difference 1.0 m). Thus, the parameters of recharge and
especially hydraulic conductivity are noticeably critical and can affect
the results and must be correctly selected during calibration of the
model.

RESULTS AND DISCUSSION

Habitats and sub-wetlands

The protected waterfowl habitats are composed of open water,
hydrophyte, helophyte, open grass and mire scrub sub-wetlands in the
investigated lake and its riparian zone.

Open water covers 41% (19.80 ha) of the lake area. Hydrophytes have
colonized 36% and helophytes 23% of it.

Sub-wetlands, situated in the riparian zone (open grass and mire
scrub sub-wetlands) cover an area that makes 87% of the lake area. A
larger part of this area (59%) is covered with mire scrubs (Fig. 6).

According to some authors (Hickey & Malecki 1997), the most
suitable habitats for the Black Tern (Chlidonias niger) are those where
50% of the territory is covered with emergent vegetation. The Little
Crake (Porzana parva) requires similar habitats (Burton & Burton
2002). The open and semi-open habitats (open water and hydrophyte
sub-wetlands) cover 30 ha in the investigated lake. The total area of
overgrown habitats (helophyte and riparian open grass sub-wetlands are
very similar in their morphometrical parameters) is 23.16 ha in the
investigated lake and its surroundings.

To conclude, the present proportion of the Lake Cedasas habitats is
similar to the proportion of vegetation cover and open water the Black
Tern (Chlidonias niger) and Little Crake (Porzana parva) need for
living.

Hydromorphological indicators

Sub-wetlands are best characterized by the lake depth and
groundwater level. According to the bathymetrical plan of the lake and
hydrogeological map of the riparian zone, isolines best fitting the
limits between various sub-wetlands were determined. These lines were
taken as the hydromorphological indicators showing the limits of
sub-wetlands.

Comparative analysis of the lake's bathymetrical plan and the
territory overgrown with macrophytes in the orthophotographical map
showed that a larger part of water areas, shallower than 1.05 m, is
totally or partially overgrown with water plants. The coincidence of the
open water area between these two maps is 99.4%. The coincidence of open
water patches dislocation between these two maps is 74.4% (Table 2, Fig.
7).

The percentage of the coincidence of the open water area and its
distribution (between the bathymetrical and orthophotographical maps) is
smaller than mentioned above, if another indicated depth is taken (Table
2). Therefore, > 1.05 m depth is essential for preservation of open
water areas in the investigated lake and it was considered as the
hydromorphological indicator of these sub-wetlands.

The hydromorphological indicator of helophyte areas corresponds to
the main helophyte distribution between the shoreline of the lake and
the 0.4 m isobath: the area coincidence is 102.2%, location coincidence
68.7% (Table 2, Fig. 7). Misalignment of the sub-wetland area and
dislocation of these sub-wetland patches between the two maps are mostly
caused by the bathymetrical map scale that gives insufficient
information at small depths. For example, there are on average less than
two measured points in 1 ha of 0-0.4 m depth.

Comparative analysis of the riparian zone of the lake (overgrown
with scrubs and grassy vegetation) and the groundwater level in this
zone showed that grassy vegetation with predominance of terrestrial
helophytes occurred mainly on the territories where groundwater depth
varied from the lake shoreline to 0.2 m. Therefore, it was supposed that
the hydromorphological indicator of open grass sub-wetlands corresponded
to the groundwater level not shallower than 0.2 m.

The last hydromorphological indicator was determined for mire scrub
sub-wetland. Comparative analysis of the dislocation of scrubs and the
groundwater level parameters showed that mire scrubs form in the areas
where the groundwater level varies between 0.2 and 1 m (Table 2, Fig.
7).

To sum up, the best coincidence in the investigated lake was
observed between the open water area discerned from the orthophotograph
and the area within 1.04 m depth isobath limits; between the area
covered by hydrophytes and the area within 0.4-1.05 m depth isobath
limits; between the area covered by helophytes and the area within 0.4 m
depth isobath limits. The best coincidence between open grass
sub-wetlands and the area within 0.2 m groundwater depth limits as well
as between the mire scrub area and the area within 0.2-1.0 m groundwater
depth limits was observed in the riparian zone of Lake Cedasas.

There are two most important hydromorphological indicators for the
protected waterfowl in the investigated lake:

1. The boundary between hydrophyte and helophyte sub-wetlands,
where the best coincidence with 0.4 m isobath was determined (Table 2).
This limit indicates the total area of open and semi-open habitats (open
water and hydrophyte sub-wetlands) that serve as nourishment areas for
the Black Tern (Chlidonias niger) and Little Crake (Porzana parva).

2. The part of the riparian zone, where groundwater depth is
greater than 0.2 m, indicating the limits for the overgrown habitats
(helophyte and open grass sub-wetlands) suitable for the Black Tern
(Chlidonias niger) and Little Crake (Porzana parva) as breeding areas.

Finally, the mire scrub sub-wetland predominates in the riparian
zone where the groundwater level declines more than 0.2 m during the dry
season, which lasts for the most part of the period of vegetation
growth. This hardly penetrable scrub belt surrounding the lake is also
very important for the Black Tern (Chlidonias niger) and Little Crake
(Porzana parva) as a protective zone, determining the safety of the
whole habitat in Lake Cedasas. It guarantees a limited use of the lake
for the recreational purposes and a quiet ambiance for the breeding
birds.

Change in the area and location of sub-wetlands

Simulation of groundwater level changes showed a decrease in the
groundwater level in the area of the dolomite quarry. After some time,
the groundwater level will become stable and will be maintained thus
during the whole of the quarry exploitation period. The 3D model results
revealed a well-defined depression of the groundwater level, reaching
even 6.5 m (very close to the results of calculations (6.8 m) performed
during environmental impact assessment; see Environmental ...
2008)--below the natural groundwater level that formerly surrounded the
quarry (Figs 8, 9). However, its real influence is observed only within
600-700 m and reaches only the Minava Stream (decrease in the
groundwater level close to the Minava Stream is only 0.2-0.3 m), flowing
from Lake Cedasas to the Vyzuona River (Fig. 1). So, the decrease in the
groundwater level reaches 10 cm in the surroundings of the investigated
lake (Figs 8, 9). Nevertheless, the Lake Cedasas water level will
decline by about 10 cm, because the precipitation will decrease and
evapotranspiration will increase (Table 1).

Based on the simulation results of groundwater changes in the
riparian zone of Lake Cedasas, it is assumed that the water budget will
change as a consequence of climate change and extraction of groundwater
in the dolomite quarry. The obtained results showed that according to
the hydromorphological indicators, the open water territory will be
divided into two separate areas and will diminish by about 25%, assuming
the impact of dolomite quarry exploitation and climate change. The area
covered by hydrophytes will increase by about 15% and the area covered
by helophytes will diminish by about 17% because of the water level
decline in the lake. Thus, the total area of Lake Cedasas will decrease
by about 8%.

However, the area of the open grass sub-wetland will decline by
about 5%, whereas the area of the mire scrub sub-wetland will increase
by as much as 28% (Table 2, Fig. 7).

In order to evaluate more precisely the water balance and water
level changes of the lake as well as the resulting habitat changes, the
FEFLOW 5.0 model (more suitable for the simulation of groundwater flows)
should be combined with other surface outflow models (FEFLOW 6.0, GSLOW
versions).

To conclude, the FEFLOW 5.0 simulation results of our study
indicate that exploitation of the dolomite quarry and further climate
change will only slightly influence the groundwater levels in the
riparian zone of Lake Cedasas. The climate and groundwater changes in
the riparian zone will change the water level of the lake. The most
negative influence on the protected waterfowl will be the reduction of
the open water area (nourishment zone) as well as of the helophyte and
open grass sub-wetlands (breeding zone), assuming that the lake level
and the groundwater level of the riparian zone will decrease by 10 cm.
The increase in the area overgrown with scrubs could, as we have
indicated earlier, have some positive protective value.

CONCLUSIONS

1. The FEFLOW 5.0 model was used for the simulation of groundwater
inflow and outflow. If these water budget elements can determine water
level changes in lakes, positive results of the application of this
model are observed (as in our case). However, if water level changes in
lakes are mainly affected by surface water inflow and outflow, the
application of the FEFLOW 5.0 model is insufficient. In this case, other
models (FEFLOW 6.0, GSLOW versions) must be used in order to simulate
the water level changes in lakes.

2. The groundwater level change in the riparian zone of Lake
Cedasas was simulated by FEFLOW 5.0. The decrease in lake water level
was predicted on the basis of the change in water balance due to climate
change. A similar change in the water level of Lake Cedasas may be
expected assuming that the water level in the lake varies similar to the
groundwater level in the riparian zone.

3. The existing proportion of open, semi-open and overgrown
habitats in Lake Cedasas and its surroundings is similar to the
proportion of vegetation cover and open water needed by the Black Tern
(Chlidonias niger) and Little Crake (Porzana parva) inhabiting the study
area.

4. According to the analysis of the most important
hydromorphological indicators (0.4 m depth in Lake Cedasas and 0.2 m
groundwater depth in the riparian zone), the most suitable habitat
structure for protected waterfowl is composed of three zones:
nourishment (open water and hydrophytes), breeding (helophytes and
riparian grass vegetation) and protective (hardly penetrable scrub belt
surrounding the lake).

5. Normally, after water extraction in the dolomite quarry, the
lake water and riparian zone groundwater levels will remain unchanged.
The assumption that precipitation and temperature will be changing in
the future, like now, and water will be extracted from the dolomite
quarry, suggests a change in the water budget. In this case, the Lake
Cedasas water level will decrease by 10 cm and the proportion of these
zones will change from 41 : 32 : 27 to 38 : 28 : 34. A negative
influence will be felt in the nourishment and breeding areas of
protected waterfowl, which tend to decrease slightly.

doi: 10.3176/earth.2013.06

Acknowledgements. We are grateful to the programme
'GEOSYSTEMS', financed by the Lithuanian Ministry of Education
and Sciences, and to the staff of Sartai Regional Park for support with
field investigations. Thanks go to the Lithuanian Geological Survey and
the Lithuanian Hydrometeorological Service under the Ministry of
Environment for material provided. We gratefully acknowledge the useful
reviews and comments of the reviewers.